Python Design Patterns - composition vs. inheritance

Bruno Desthuilliers bruno.42.desthuilliers at
Fri Nov 16 19:14:53 CET 2007

snewman18 at a écrit :
> It's hard to apply some of the available
> material's examples to Python since a lot of the documentation I find
> is specific to implementations in lower-level languages and don't
> apply to Python. 

Fact is that quite a few design patterns are mostly workaround the 
low-level and lack of dynamism of the C++/Java family. In higher-level 
dynamic languages like Python, Ruby etc, they are not "patterns", they 
are idioms with a good builtin support !-)

> (such as the Strategy pattern?)

The "Strategy" pattern is mostly about delegating (part of) a behaviour 
to someone else. As such, it can be found (one way or another) in almost 
any language - even in C with function pointers (look at how the C 
stdlib sort() function works...).

> My understanding was that using __getattr__ was either called
> delegation or a Command pattern,

Now we have proper computed attributes, __getattr__ is mainly used for 
delegation, yes. But some years ago, it was also the way to implement 
computed attributes.


>> Of course there are just reasons to create such delegation methods - if
>> for example the property/method is of general interest to the Pet, but
>> implemented by means of the owner. But I've got difficulties even to
>> imagine such thing, at least in your actual example.
> Yeah, I was struggling to come up with a decent example - a pet's
> address was about the best example of a delegated property I could
> think of. If someone has a better set of objects that make sense, let
> me know and I'll probably feel less foolish asking.

class Forbidden(Exception): pass

class Interceptor(object):
   def __init__(self, obj, allow_access):
     self.__obj = obj
     self.__allow_access = allow_access

   def __getattr__(self, name):
       if self.__allow_access(self.__obj, name):
           return getattr(self.__obj, name)
           raise Forbidden

As Carl mentioned, real-life code usually uses more technical objects 
than domain objects.

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